That iiiiis a toughie.
First I'd have to say Brent Hinds and Troy Sanders of Mastodon. The first two, because they've got fantastic scream/growl ranges and amazing clean too.
Second would have to be Bruce Dickinson of Iron Maiden, and if you don't know why, go listen to Maiden. NAOW.
Then in no particular order, Corey Taylor of Slipknot/Stone Sour, Burton C Bell of Fear Factory, Tom Araya of Slayer, Roger Daltrey of the Who, Billie Joe Armstrong from Green Day (OLD STUFF, NO FLAMIN') and Steven Wilson of Porcupine Tree.